A Dual Isotope Conundrum: Unraveling a Cryptic Subsurface N Cycle

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Nitrogen is an essential macronutrient that largely regulates primary productivity across a host of global ecosystems. Through human activities, global turnover of N has more than doubled, resulting in massive and widespread alteration of fundamental ecosystem dynamics leading to pronounced impacts on both human and ecological health. An accurate understanding of the (bio)geochemical cycling of N is, therefore, of central importance to enabling the prediction of its fate in the environment. This project aims to quantify linkages between the N and Fe cycles in the subsurface terrestrial biosphere, through a comprehensive examination of the biotic and abiotic interactions between NO3- (and NO2-) and Fe and their influence on the dual isotopic composition (e.g., delta 15N and delta 18O) of NO3- in the environment. Specifically, there exists a fundamental discrepancy of coupled N and O isotope effects between microbial culture-based isotopic studies of denitrification and field-based isotopic measurements of groundwater NO3- loss under anaerobic conditions (in which denitrification is widely implicated). We suggest that this discrepancy is the result of a ubiquitous, yet largely unrecognized, coupling of abiotic and/or biotic Fe cycling with both reductive and anaerobic oxidative N cycling pathways and have preliminary data supporting this hypothesis. Testing of this hypothesis will help reveal the underlying causes of this apparent isotope conundrum and will have important implications for improving our understanding of the coupling of subsurface N, C and Fe cycling in the terrestrial subsurface. This project will also support the development of an annual 2-day teacher-training workshop in support of Science, Technology, Engineering and Math education in Massachusetts. We will partner with the Waquoit Bay National Estuarine Research Reserve (WBNERR) to conduct a workshop (about 15 high school teachers) focused on wastewater issues in watersheds and nitrogen cycling in coastal ecosystems. The workshop will provide participants with an understanding of the natural nitrogen cycle, human impacts and environmental effects and approaches for minimizing the impact of anthropogenic N loading in local watersheds and fragile coastal ecosystems.